1. Field of the Invention
The present invention generally relates to a disk drive, and more particularly to a disk drive with a rotary actuator-based positioning mechanism, mounted so that it can rotate freely about an axis normal to the plane of actuator motion.
2. Description of the Related Art
As track density continues to grow, the vibration-induced track following error component is expected to become even more critical to the operation of a disk drive. To achieve the expected future demand of 20 Gbits-per-sq. inch areal density, a track density of 50 thousand tracks per inch (kTPI) is needed compared to the present 20 kTPI design point.
However, at high TPI (e.g., above 20 kTPI), the in-plane (theta coordinate) rotational vibration of a disk drive, henceforth referred to as “theta-dynamics”, will emerge as a dominant track misregistration (TMR) component. This is because a hard disk drive (HDD) with a rotary actuator system is highly sensitive to in-plane rotational vibration of its baseplate. A solution to the vibration challenge can be developed along several disciplines, ranging from novel mount systems to sophisticated servo algorithms. However, prior to the present invention, none has been satisfactory, let alone optimal.
A computer system may include one or more disk drives where each drive contributes to the total vibration environment. In addition, the computer system itself may be subjected to external seismic excitations. Head positioning accuracy in a disk drive is prone to both self-generated vibration and to vibration generated by another disk drive or other peripherals attached to the same mounting structure. A disk drive with a rotary actuator mechanism produces self-vibration due to mechanical motion resulting from: 1) spindle mass imbalance producing a periodic vibration of the baseplate; and 2) actuator seek reaction torque producing transient dynamics of the baseplate.
These two components (e.g., spindle mass imbalance and actuator seek reaction torque) also produce vibration of the computer chassis which in turn affects other storage devices attached to the same chassis. This effect is referred to as “emitted vibration.” Likewise, the drive of interest is subject to “emitted vibration” produced by the neighboring units/drives which can be viewed as an externally-imposed vibration component.
Hence, a solution to three vibration challenges (e.g., self-generated vibration, external vibration, and emitted vibration) is mandatory for a high performance computer storage configuration. Hitherto the present invention, such a solution has not been found.
Further, it is noted that the present generation of 2.5″ and 3.5″ hard disk drives (HDDs) are designed to operate in portable and desk-top/server environments, respectively. To reduce cost and weight of a computer system, manufacturers typically fabricate the HDD mounting frame utilizing thin structural members. Therefore, a computer frame is a compliant object which makes it susceptible to vibration. Such a mounting configuration makes a disk drive vulnerable to vibrations excited by internal or external sources. A head positioning servo system in an HDD performs three critical tasks. First, it moves the head to the vicinity of a target in a minimum time using a velocity servo under a seek mode. Next, the servo system positions the head on the target track with minimum settle-out time using a position controller without an integrating term in it. Finally, the servo system enters the track-follow mode with a proportional-integral-derivative-type (PID) position controller. During the seek mode, maximum rotational acceleration torque followed by a deceleration torque is imparted by a voice coil motor (VCM)-based actuator. The corresponding reaction torque on the baseplate causes transient rotational vibration that can be detrimental to the positioning accuracy of the read/write heads.
Present 3.5″ disk drives have reached 20 kTPI, and after year 2000 it is expected to grow beyond 25 kTPI. As mentioned above, a major obstacle to raising the track density is inadequate head positioning accuracy in the presence of vibration disturbances. Due to exponential growth in TPI, positioning the read/write elements over a track has become a major challenge. Conventional servo control system requires continuous innovations to perform well under increasingly difficult operating conditions.
It is noted that the mechanical components such as spindle motor assemblies are not perfectly mass-balanced, and during operation they produce harmonic vibration. Harmonic vibration excitation produces both a linear and a rotational oscillatory motion of the whole HDD system. At a 15 kTPI design point, a rotational oscillatory motion of a track with respect to the actuator pivot of about 0.01 thousandth of an inch (e.g., about 0.25 micrometer) corresponds to 15% of the track pitch. When not compensated, a track following error of 15% of track pitch can be detrimental to a disk drive's “soft” and “hard” error rate performance. The positioning error due to this internally-produced periodic vibration can be solved using a servo method proposed in U.S. Pat. No. 5,608,586, incorporated herein by reference.
By using special shock and vibration isolation mount design, the rotational oscillatory components due to internal spindle forcing can be minimized as disclosed in U.S. Pat. No. 5,400,196, incorporated herein by reference.
However, a mount design optimized to decouple internal spindle vibration as covered by U.S. Pat. No. 5,400,196, incorporated herein by reference, remains susceptible to external input vibration. By deploying the isolation mounts along a polygon satisfying a particular set of criteria defined by Japan Patent No. 2,565,637, incorporated herein by reference, the external vibration inputs generating rotational vibration on an HDD can be minimized. In co-pending U.S. patent application Ser. No. 09/119,184, commonly assigned and having IBM Docket No. YO9-98-231, a method of neutralizing the reaction by generating a counter torque using a secondary actuator was proposed.
However, these above-mentioned methods are deficient in that each solves only a subset of the three vibration challenges of an HDD, and none of them provides a simple, low-cost solution to the seek-induced transient dynamics. Thus, an HDD with a novel mounting frame that is more robust against vibration can yield a competitive product, but hitherto the present invention, such a problem has not been recognized, nor has a structure effectively addressing such a solution been developed
Using sensors, servo algorithms, and inertial force generators, undesirable vibration of a mechanical device, such as an HDD, can be controlled. Previously, using shock isolating rubber mounts, storage devices have been protected from linear shock input. However, due to sway, space requirements, and gradual improvements in shock enhanced storage component design, explicit shock isolation of disk drives has no longer been pursued by manufacturers. Removal of traditional shock and vibration isolation mounts (e.g., see U.S. Pat. No. 5,349,486) was further accelerated by dynamic problems encountered during the operation of a drive (e.g., see U.S. Pat. No. 4,967,293). Since 1990, the storage industry has moved away from shock isolation design. In today's market, it is believed that no disk drive is manufactured with a shock and vibration isolation system. Previously, use of a shock isolation system protected an HDD from shock handling, but it actually degraded the linear vibration problem as well as the settle-out problem. Further, traditional isolation systems use damping materials that are not good heat conductors.
Thus, in view of all of the foregoing problems, hitherto the invention, there has been no system in which the plurality of vibrational components have been effectively compensated and in which a seek induced settle-out dynamics problem has been solved using a unique rotational mount concept.